The present invention is directed generally to a fiber optic device, and more particularly to a multiport circulator.
Non-reciprocal devices are used in microwave and optical communications for selectively directing signals from one port to another. With the growth in fiber optic communications, there is an increasing demand for non-reciprocal components that are suitable for use with fiber optic systems. For example, isolators are used for increasing the stability of the frequency and power produced by single mode semiconductor lasers by reducing the power of light feeding back into the laser. As fiber optical systems become more sophisticated, for example with the advent of wavelength division multiplexing (WDM), there is an increased need for advanced components such as optical circulators for use in, for example, multiplexing/demultiplexing, add/drop multiplexing and bidirectional transmission.
In complex fiber optic systems, there may be a multiplicity of fibers, each requiring a similar function. For example, a number of fibers may carry WDM signals that require demultiplexing. Some current demultiplexing methods require a separate demultiplexer associated with each fiber. It may be more convenient and cost-effective to provide parallel, shared capabilities, where each WDM channel signal shares some or all of the components of the demultiplexer with other WDM channels.
Present approaches to circulator design do not easily allow for scaling a circulator up for use with several channels. In addition, many circulators use a large number of components, which results in devices that are complex to align, expensive to assemble, and high in reflective loss.
In order to fit into a high density distribution frame, fiber optic components tend to be small. Moreover, in some space-limited applications, devices need input and output ports on one side. Therefore, there is also a requirement that a circulator be small so as to be compatible with the components of the rest of the fiber optic system.
Many circulator designs include the use of optical components having optical faces that are epoxied to each other. The long-term durability of the optical epoxy is uncertain, and so there is concern that such components may have a shorter mean-time-to-failure than other components that do not have epoxied faces.
Many circulators employ retardation waveplates for rotating the polarization direction of the optical beams. However, the thickness of a waveplate is accurately set for a particular wavelength, and deviation from the set wavelength compromises the polarization rotation properties of the waveplate. Therefore, the use of a waveplate in a circulator reduces the bandwidth of the isolation between ports, and constricts use of the device to a narrow wavelength range.
There exists a need to provide a circulator that has an epoxy-free optical path, is compact, and has a small number of components so that fabrication is less complex, and reflective losses are reduced. There is also a need to provide the ability to scale up the circulator to allow a larger number of inputs and outputs and also to provide the ability to operate with different channels in parallel. There is also a desire to avoid the use of retardation waveplates in the circulator, in order to obtain a wide operating wavelength range.